1. Field of the Invention
The present invention relates to a separator for a fuel cell and a fuel cell provided with the same.
2. Description of the Prior Arts
Various separators for a fuel cell have conventionally been proposed. Many separators made of a metal have been designed so far in addition to those made of carbon. It is needless to say that a strength not less than a specified amount is required in view of its structure and material for withstanding the use as the separator. However, the required point as the separator is not only the quality with respect to the strength. A variation per hour due to the deterioration of the material forming the separator must be small even under an environment where a power generation is executed by causing a chemical reaction between two kinds of gas under high temperature. The deterioration of the separator has an adverse effect on an electrolyte film or a catalyst, resulting in reducing the power generation efficiency.
A technical expert involving the separator for a fuel cell has repeated trials and errors to find a suitable material for the separator or for the surface treatment so far. As a result, several kinds of material have been known to be suitable to some degree to the separator. One of them is disclosed in Japanese Unexamined Patent Application No. HEI 10-228914. This application discloses that a press molding is performed to a stainless steel to form an evagination section for passing a gas and a gold plating is partially provided on the surface of the evagination section that is in contact with each electrode.
A gold plating is stabilized from a viewpoint of a material, and has a small variation per hour and also has a reduced contact resistance in case where it comes into contact with a cathode electrode. Therefore, it is excellent when used as the surface treatment on the separator at the cathode side. However, when the gold plating is used as the partial surface treatment on the separator at the anode side, an elution of metal ion from the section having no gold plating cannot be avoided, thereby entailing a problem of deteriorating a performance of the fuel cell. Further, when the gold plating is provided all over the separator at the anode side because of this problem, high cost of the separator becomes a problem that cannot be avoided.
On the other hand, Japanese Unexamined Patent Application No. 2000-294256 discloses a separator for a fuel cell utilizing a ferrite stainless. This separator can prevent the elution of metal ion when coming in contact with the anode electrode, but it is impossible to prevent the increase in contact resistance at the cathode electrode side, thereby also not avoiding the deterioration in the performance of the cell. As described above, any one of the separators designed by the conventional technique does not show excellent performance even in case where it is arranged at the anode electrode side and at the cathode electrode side.
The inventor of the present invention has repeated research on the separator for a fuel cell and found that a mechanism causing a performance deterioration of the fuel cell due to the deterioration of the separator is different between at the anode electrode side and at the cathode electrode side. Specifically, at the anode electrode side, metal ion of the material of the separator is eluted and mixedly included as a contaminant into an electrolyte film or catalyst layer, thereby causing the performance deterioration of the fuel cell. Further, condensed water contained in a hydrogen gas introduced to the anode electrode side becomes a path of metal ion, thereby spreading the elution of metal ion. On the other hand, it has been found that, at the cathode electrode side, an oxide coating layer is grown on the surface of the separator to thereby increase the contact resistance between the cathode electrode and the separator, which induces the performance deterioration of the fuel cell.